During the operation of a telephone system, it is often necessary to test the conditions of a telephone line, particularly those referred to as subscriber loops which connect terminal sets to a central office. In the past, this has been accomplished by connecting various configurations of test equipment to the line to separately determine each of the wanted parameters. An extensive analysis would involve measuring at least thirteen separate parameters including any spurious a-c voltage, d-c voltage, leakage resistance and line capacitance between the tip and ground, ring and ground, and tip and ring of the subscriber's loop, as well as the frequency of the spurious a-c voltage. It will be readily apparent that determining each of these parameters separately in order to evaluate the state of the subscriber loop was a long and tedious operation.
Richard Scott Hoppough's U.S. Pat. No. 4,028,507 issued June 7, 1977, discloses an improved apparatus for measuring the various parameters, which entails forcing the tip and ring to several different sets of potentials with relation to ground, and then measuring the resultant currents flowing therethrough. From these measurements, the various parameters can then be calculated. However with this arrangement separate sets of d-c and a-c voltages must be applied to measure the respective leakage resistances and the line reactances (primarily capacitive). In addition, filters are required to reject any spurious a-c signals on the line during the a-c measurements.